Review of the immune system

CHAPTER 67


Review of the immune system


Life is a constant battle, and the immune system is the army that helps us prevail. This system protects us from invading organisms (viruses, bacteria, fungi, and parasites) and can destroy cancer cells before they destroy us. Unfortunately, the army does not always act in our best interest: It can attack transplanted organs and tissues and, when it runs amok, can turn on the cells it normally protects.


To study the immune system, we begin with an overview. After that, we discuss the two major types of specific immune responses: antibody-mediated immunity (humoral immunity) and cell-mediated immunity.




Introduction to the immune system


Our objective in this section is to establish an overview of immune system components and how they function. Much of the information introduced here is amplified later.



Natural immunity versus specific acquired immunity


Our bodies can mount two types of immune responses, referred to as natural immunity (innate or native immunity) and specific acquired immunity. Factors that confer natural immunity include physical barriers (eg, skin), phagocytic cells, and natural killer cells. All of these factors are present prior to exposure to a particular infectious agent and all respond nonspecifically. In contrast, specific acquired immune responses occur only after exposure to a foreign substance. The foreign substances that induce specific responses are called antigens, and the objective of the immune response is to destroy them. With each succeeding re-exposure to a particular antigen, the specific immune response to that antigen becomes more rapid and more intense. Specific immune responses are possible because certain cells of the immune system (T lymphocytes and B lymphocytes) possess receptors that can recognize individual antigens. In this chapter, our focus is on specific acquired immunity, not on natural immunity.




Introduction to cells of the immune system


Immune responses are mediated by several types of cells, some of which play a bigger role than others. The major actors are the lymphocytes (B cells, cytolytic T cells, helper T cells), macrophages, and dendritic cells. Accessory cells include neutrophils and basophils. With the exception of some dendritic cells, all of the cells involved in the immune response arise from pluripotent stem cells in the bone marrow (Fig. 67–1) and, for at least part of their life cycle, circulate in the blood. Defining characteristics of individual immune system cells are summarized in Table 67–1.



image
Figure 67–1  Maturation of blood cells.
With the exception of platelets and erythrocytes, all of the mature blood cells shown participate in immune responses. However, only cells of lymphoid origin (cytolytic T cells, helper T cells, B cells) possess receptors that can recognize specific antigens. (CFU = colony-forming unit.)
1Monocytes that have moved into tissues are called macrophages.
2Basophils that have moved into tissues are called mast cells.





Helper T lymphocytes (helper T cells, CD4 cells).

Helper T cells contribute to the immune response in three ways: (1) they have an essential role in antibody production by B cells, (2) they release factors that promote delayed-type hypersensitivity (DTH), and (3) they participate in the activation of cytolytic T cells. Specificity of helper T cells is achieved through highly specific cell-surface receptors that recognize individual antigens. Like other lymphocytes, helper T cells circulate in the blood and lymph. Helper T cells carry CD4 (cell differentiation complex 4) marker molecules on their surface, and hence are referred to as CD4 cells.


The term helper is somewhat misleading, in that it connotes a useful but dispensable role. Nothing could be further from reality. Helper T cells are not simply nice to have around, they are absolutely required for an effective immune response. The critical nature of their contribution—and the grim consequences of their absence—are manifested in people with HIV/AIDS: Helper T cells are the immune cells that HIV attacks. Because of helper T-cell loss, AIDS patients are at high risk of death from opportunistic infection.



Macrophages.

Macrophages begin their existence in the bone marrow, enter the blood as monocytes, and then infiltrate tissues, where they evolve into macrophages. Macrophages are present in all organs and tissues.


The primary function of macrophages is phagocytosis (ie, ingestion of microbes, other foreign material, and cellular debris). In their role as phagocytes, macrophages are the principal scavengers of the body. Although their major job is phagocytosis, macrophages also have an important role in specific acquired immunity, natural immunity, and inflammation.


In specific acquired immunity, macrophages have three functions: (1) they are required for activation of T cells (both helper T cells and cytolytic T cells), (2) they are the final mediators of DTH, and (3) they phagocytize cells that have been tagged with antibodies. Of these three immune-related roles, activation of T cells is arguably the most critical. When performing this function, macrophages are referred to as antigen-presenting cells (APCs). Because antigen presentation is an absolute requirement for specific immune responses (see below), you can appreciate how important macrophages are.







Antibodies


Antibodies are a family of structurally related glycoproteins that mediate humoral immunity. The most characteristic feature of antibodies is their ability to recognize and bind with specific antigens. Alternative names for antibodies are immunoglobulins and gamma globulins.


All antibodies are produced by B lymphocytes. Some of the antibodies that B cells produce are retained on the surface of the B cell, where they serve as the receptors whereby B cells recognize specific antigens. However, most of the antibodies that B cells produce are secreted from the cell, after which they bind to their specific antigen, thereby initiating the effector phase of humoral immunity. The process of antibody production is discussed in detail below.


All antibodies are composed of units that have the same basic structure. As shown in Figure 67–2, antibodies have four chains: two heavy chains and two light chains. Disulfide bridges connect the four chains to form a unit. Each heavy chain and each light chain has two regions, one in which the sequence of amino acids is constant and one in which the sequence is highly variable. The variable regions form the antigen-binding site.


image
Figure 67–2  Antibody structure.
The basic antibody structure depicting heavy and light chains is shown on the left. Variable regions of the heavy and light chains, which form the antigen-binding site, appear in green. As shown on the right, papain digestion of antibodies produces two types of fragments: Fab fragments, which retain the ability to bind antigen, and Fc fragments, which do not bind antigen and tend to crystallize in the test tube.

There are five classes of antibodies (immunoglobulins), known as IgA, IgD, IgE, IgG, and IgM. All are constructed from the same basic parts just described. However, the heavy chains differ for each class. Primary functions of the five classes are summarized in Table 67–2.



TABLE 67–2 


Functions of Antibody Classes




















Class Function
IgA
IgD
IgE
IgG
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Jul 24, 2016 | Posted by in NURSING | Comments Off on Review of the immune system

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